08 Quadrilaterals

Quadrilaterals
Geometry
Chapter 8
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
This Slideshow was developed to accompany the textbook
 Larson Geometry
 By Larson, R., Boswell, L., Kanold,T. D., & Stiff, L.
 2011 Holt McDougal
Some examples and diagrams are taken from the textbook.
Slides created by
Richard Wright, Andrews Academy
[email protected]
8.1 Find Angle Measures in Polygons

Polygon
 Closed figure made of straight segments

Diagonal
 Segment that joins nonconsecutive vertices
8.1 Find Angle Measures in Polygons

All polygons can be separated into triangles

The sum of the angles of a triangle is 180°
For the pentagon, multiply that by 3

Polygon Interior Angles Theorem
Sum of the measures of the interior angles of a n-gon is (n2)180°
= − 2 ⋅ 180°
Sum of the measures of the interior angles of a quadrilateral is
360°
8.1 Find Angle Measures in Polygons

The coin is a regular 11-gon. Find the sum of the measures of
the interior angles.

The sum of the measures of the interior angles of a convex
polygon is 1440°. Classify the polygon by the number of sides.
8.1 Find Angle Measures in Polygons

Find m∠T
8.1 Find Angle Measures in Polygons
Polygon Exterior Angles Theorem
Sum of the measures of the exterior angles of a convex polygon
360°

What is the measure of an exterior angle of a regular pentagon?
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What is the measure of an interior angle of a regular pentagon?
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510 #2-34 even, 40-46 even = 21
Answers and Quiz
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8.1 Answers

8.1 Homework Quiz
8.2 Use Properties of Parallelograms

On scrap paper draw two sets of parallel lines that intersect
each other.

Measure opposite sides. How are opposite sides related?
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Measure opposite angles. How are opposite angles related?
8.2 Use Properties of Parallelograms

Definition of parallelogram
 Quadrilateral with opposite sides parallel
Opposite sides of parallelogram are congruent
Opposite angles of a parallelogram are congruent
8.2 Use Properties of Parallelograms
Consecutive

angles in a parallelogram are supplementary
Remember from parallel lines (chapter 3) that consecutive
interior angles are supplementary
Diagonals of a parallelogram bisect each other


Draw diagonals on your parallelogram
Measure each part of the diagonals to see if they bisect each
other.
8.2 Use Properties of Parallelograms


Example:
Find x, y, and z
y
20°
z°
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

x = 70
y = 42
z = 20
42
x°
8.2 Use Properties of Parallelograms

Find NM
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Find m∠JML
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Find m∠KML
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518 #4-28 even, 32, 36, 43, 44, 46-56 even = 23
Extra Credit 521 #2, 4 = +2
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Answers and Quiz

8.2 Answers

8.2 Homework Quiz
8.3 Show that a Quadrilateral is a
Parallelogram
Review
 What are the properties of parallelograms?
 Opposite sides parallel
 Opposite sides are congruent
 Opposite angles are congruent
 Diagonals bisect each other
8.3 Show that a Quadrilateral is a
Parallelogram

If we can show any of these things in a quadrilateral, then it is a
parallelogram.
• If both pairs of opposite sides of a quad are parallel, then it is a
parallelogram (definition of parallelogram)
• If both pairs of opposite sides of a quad are congruent, then it is a
parallelogram.
• If both pairs of opposite angles of a quad are congruent, then it is a
parallelogram.
• If the diagonals of a quad bisect each other, then it is a parallelogram.
• If one pair of opposite sides of a quad is both parallel and congruent,
then it is a parallelogram.
8.3 Show that a Quadrilateral is a
Parallelogram
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Examples: Is it a parallelogram?
6 cm
6 cm
8.3 Show that a Quadrilateral is a
Parallelogram

In quadrilateral WXYZ, m∠W = 42°, m∠X = 138°, m∠Y = 42°.
Find m∠Z. Is WXYZ a parallelogram?
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Find x so that MNPQ is a parallelogram.
8.3 Show that a Quadrilateral is a
Parallelogram
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526 #4-30 even, 34, 36, 39, 43-47 all = 22
Answers and Quiz

8.3 Answers

8.3 Homework Quiz
8.4 Properties of Rhombuses, Rectangles,
and Squares




All of these are parallelograms
Rhombus
 Four =̃ sides
Rectangle
 Four right ∠s
Square
 Rhombus and Rectangle
 Four =̃ sides
 Four right ∠s
8.4 Properties of Rhombuses, Rectangles,
and Squares
8.4 Properties of Rhombuses, Rectangles,
and Squares

For any rectangle EFGH, is it always or sometimes true that
≅ ?

A quadrilateral has four congruent sides and angles. Classify
the figure.
8.4 Properties of Rhombuses, Rectangles,
and Squares
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Diagonals
Rhombus: diagonals are perpendicular
Rhombus: diagonals bisect opposite angles
Rectangle: diagonals are congruent
8.4 Properties of Rhombuses, Rectangles,
and Squares
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ABCD is a rhombus
Find m∠AED
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Find DB
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Find AC
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8.4 Properties of Rhombuses, Rectangles,
and Squares
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QRST is a rectangle with QS = 10
Find m∠QPR
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Find RP
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Find RS
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537 #2-52 even, 60-70 even = 32
Extra Credit 540 #2, 5 = +2
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
Answers and Quiz
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8.4 Answers

8.4 Homework Quiz
8.5 Use Properties of Trapezoids and Kites
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Trapezoid
 Quadrilateral with exactly one pair of parallel sides

If the legs are =̃, then the trap is isosceles
8.5 Use Properties of Trapezoids and Kites
If isosceles trapezoid, then each pair of base angles is =̃.
If isosceles trapezoid, then diagonals are =̃.

The converses are also true
8.5 Use Properties of Trapezoids and Kites

Midsegment of a Trapezoid
 Segment connecting the midpoints of each leg
Midsegment Theorem for Trapezoids
The midsegment of a trapezoid is parallel to the bases and its
length is the average of the lengths of the bases.
1
= 1 + 2
2
8.5 Use Properties of Trapezoids and Kites

If EG = FH, is trapezoid EFGH isosceles?

If m∠HEF = 70° and m∠FGH = 110°, is trapezoid EFGH isosceles?

In trapezoid JKLM, ∠J and ∠M are right angles, and JK = 9 cm. The
length of the midsegment of trapezoid JKLM is 12 cm. Find ML.
8.5 Use Properties of
Trapezoids and Kites

Kites
 Quadrilateral with 2 pairs of consecutive congruent sides
If kite, then the diagonals are perpendicular.
If kite, then exactly one pair of opposite angles are
congruent.
8.5 Use Properties of Trapezoids and Kites

In a kite, the measures of the angles are 3x°, 75°, 90°, and 120°.
Find the value of x.

546 #4-32 even, 38, 44-48 all = 21
Answers and Quiz

8.5 Answers

8.5 Homework Quiz
8.6 Identify Special Quadrilaterals
8.6 Identify Special Quadrilaterals
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Quadrilateral DEFG has at least one pair of opposite sides
congruent. What types of quadrilaterals meet this condition?
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Give the most specific name for the quadrilateral.
8.6 Identify Special Quadrilaterals
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Give the most specific name for the quadrilateral.
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A student knows the following information about quadrilateral
MNPQ: ‖, ≅ , and ∠ ≅ ∠. The student concludes
that MNPQ is an isosceles trapezoid. Why is this wrong?

554 #3-12 all, 14-30 even, 38, 40, 44-50 even = 25
Extra Credit 557 #2, 4 = +2
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Answers and Quiz
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8.6 Answers

8.6 Homework Quiz
8.Review
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564 #1-18 all = 18